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This paper presents, for the first time, a novel rat-race coupler design with tunable power dividing ratio. Wide tuning ratio is accomplished by the use of two tuning diodes and a single control voltage. Ideal port isolation and return loss characteristics, evaluated at the center frequency of operation, are proven for all dividing ratios. The structure is simple to construct and involves 50-ohm lines only. For verification, the simulated and measured results of a 1-GHz rat-race coupler implemented on microstrip are shown.